The present invention relates to a fluorine-containing cation exchange membrane for electrolysis and a process for producing alkali metal hydroxide by using the same.
In recent years, alkali electrolysis by an ion exchange membrane method to produce an alkali metal hydroxide and chlorine by electrolyzing an alkali metal chloride aqueous solution by means of a fluorine-containing resin ion exchange membrane as a membrane for electrolysis, has been internationally spread since it enables the production of an alkali metal hydroxide of a high purity at a low energy consumption in comparison with a conventional method.
In the early stage of such alkali electrolysis by the ion exchange membrane method, a fluorine-containing ion exchange membrane having sulfonic acid groups as ion exchange groups was used, but the ion exchange membrane has been recently replaced by a cation exchange membrane having carboxylic acid groups as ion exchange groups since the former ion exchange membrane could not raise an electric current efficiency. As this result, the electric current efficiency of electrolysis has reached about 93 to 97% which is almost completely satisfactory for industrial use.
However, although the above mentioned carboxylic cation exchange membrane achieves an excellent electric current efficiency for a long term, it has been found that it is suitable only for producing an alkali metal hydroxide of a concentration of up to about 35% by weight. According to the research of the present inventors, when the concentration of an alkali metal hydroxide to be produced has exceeded about 35% by weight, there has been a phenomenon that the electric current efficiency has been gradually lowered during a long operation term of from one month to one year although the current efficiency has been high at the initial stage of operation. Thus, the carboxylic cation exchange membrane is not always suitable for industrially producing an alkali metal hydroxide of such a high concentration.
On the other hand, there has been proposed a process for producing an alkali metal hydroxide of a concentration of at least 30% by weight by using a sulfonic cation exchange membrane of a fluorine-containing resin, but the electric current efficiency by this process is also lowered when the operation is continued for a long term. That is, the electric current efficiency at the initial stage is from 91 to 93%, but it is lowered to from 87 to 90%.
Furthermore, U.S. Pat. No. 4,455,210 discloses a process for producing an alkali metal hydroxide of a high concentration by using a cation exchange membrane comprising a fluorine-containing polymer film having sulfonamide acid groups, the cathode side of which is laminated with a fluorine-containing polymer film having sulfonic acid groups. However, this process has disadvantages not only that the electric current efficiency at the initial stage is low but also that the electric current efficiency is further lowered since the sulfonic acid group layer on the cathode side is peeled when the operation is conducted for a long term. Furthermore, Japanese Unexamined Patent Publication No. 105598/1977 discloses a process for producing an alkali metal hydroxide by using an ion exchange membrane comprising a fluorine-containing polymer having sulfonic acid groups, at the cathode side of which a porous membrane of a fluorine-containing polymer having sulfonic acid groups is disposed. In this case, since a three compartment type electrolysis system having two membranes is employed, the electrolysis voltage is high, and it is necessary for obtaining a high electric current efficiency to supply a dilute alkali solution to the intermediate compartment by pressure, thus the electrolysis operation being very complicated.
Still further, in the case of the conventional electrolysis by an ion exchange membrane method, an allowable amount of a heavy metal ion such as calcium in an alkali metal chloride aqueous solution starting material must be restricted to at most 0.05 ppm, particularly to at most 0.01 ppm, thereby the manufacturing cost being very high.